With a defective or missing combustion of the cylinder charge of an internal combustion engine, the cylinder charge is, under some circumstances, exothermally converted in the downstream catalytic converter. If this fault occurs with a certain frequency, this leads to irreversible damage of the catalytic converter by overheating.
In connection with the endangerment of the catalytic converter, it is known from U.S. Pat. No. 5,231,869 to detect misfires on the basis of an evaluation of crankshaft speeds within predetermined crankshaft angle sectors. In accordance with this publication, the fuel metering to the affected cylinder is interrupted when misfires occur. Furthermore, the misfire detection is intended, according to this publication, to be deactivated in specific operating conditions, inter alia, for a short time span after the start of the engine.
Especially with the starting of an engine with a hot catalytic converter, an overheating of the catalytic converter can occur when there are starting difficulties because of an air/fuel mixture not combusted in the cylinder. Imparting movement to a vehicle (for example, by allowing a vehicle to roll downhill in order to start the engine by releasing the clutch) is especially critical because, in this case, large quantities of uncombusted air/fuel mixture can reach the catalytic converter.
Accordingly, at the start and especially when imparting movement to the vehicle, there is the requirement to prevent supplying an uncombusted air/fuel mixture to the catalytic converter.
An expansion of the misfire detection disclosed in U.S. Pat. No. 5,231,869 to the time region after a start or during a start attempt has been shown to be especially inadequate especially in the critical case of imparting movement to the vehicle.
It is an object of the invention to provide a method with which damage to a catalytic converter can be avoided even during start difficulties, especially in the critical case of imparting movement to a vehicle as described above.
The method of the invention is for protecting a catalytic converter in the exhaust gas of an internal combustion engine in a starting operation of the engine. The method includes the steps of: forming an index for the quantity of an air/fuel mixture which will first react exothermally in the catalytic converter downstream of the engine; comparing the index to a predetermined threshold value; and, cutting off the metering of fuel when the threshold value is exceeded.
The invention reduces the danger of damage to the catalytic converter when there are starting difficulties.
The invention requires no additional complexity with respect to the apparatus when using the signal detection which is anyway provided for detecting combustion misfires. This signal detection is based on the fluctuations of the angular velocity of the crankshaft. The combustions in the time region after the start can be detected via the changed signal detection and signal processing. The change of the signal detection supplies an advantageous possibility for carrying out the invention.